In the fields of fiber optics and lens technology, there exist well developed techniques for enhancing the angular gathering range of radiation, widening the so-called "angle-of-acceptance" of a lens, window, or conduit. Moreover, one focus of these technologies, has been to assure that all accepted radiation, including radiation oblique to the axis of the device, is internally reflected repeatedly to bring it to the detector or receiver.
However, there exist applications which require the detection of radiant energy, especially visible and near-visible light, in which a contrary goal is sought. Exemplary applications are those which involve feeble and distant light sources. The aim is to filter out radiation which is not propagating parallel to the line of sight of the detecting device. These applications include long exposure astrophotography, and closely related methods of navigation and astrogation that make use of the light of feeble stars as a "navigational grid." Other applications concern such low level radiation detection schemes as: night vision for military and police surveillance, high resolution aerial photography, and even studies by biologists of the nocturnal habits of animals.
The devices used in these applications are generally already operating near a fatally low signal-to-noise ratio. A good portion of the optical noise entering such systems is in the form of stray radiation coming, not from the desired target, but from the background. At very low radiation levels, the intensity of this background radiation may equal or exceed that emanating from the target, and will certainly have an adverse effect on image resolution. When the target is illuminated by the observer, as, for example, in infra-red reconnaissance, some filtering of the back-scatter noise can be accomplished by adjusting the system geometry or by "gating" the detector to take advantage of the time difference between back-scatter and light reflected from the target. When naturally occuring low radiation levels are used, however, neither ploy can be effective.
A complex off-axis filter can be designed and constructed which is composed of layers of glass formulations having differing indices of refraction in a gradient which leads away off-axis light rays. Such devices suffer from distortion and breakage due to the differing coefficients of thermal expansion which generally accompanies differing indices of refraction.
Another solution would be to provide a peripheral jacket or cladding of light absorbing material with an index of refraction equal to that of the core of the filter. The problem is that the index of refraction is dependent on the frequency of radiation or light and the variation is not the same for different glass compositions. It is impractical to match indices for clear glass and black or absorbing glass over the frequency spectrum to avoid internal reflection.
These and other limitations of the prior art and alternative devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the present invention to provide a radiation conduit which reduces the transmittance of off-axis components of the available radiation.
Another object of the invention is to provide a contrast enhancing component for a low level radiation imaging device.
A further object of the invention is to provide a contrast enhancing aperture for a low level radiation imaging device which allows long term exposure of the detector to radiation from the target without interference from stray radiation.
Another object of the present invention is to provide an off-axis radiation filter having substantially the same coefficient of thermal expansion throughout.
A still further object of the invention is to provide an off-axis radiation filter which is effective for broad bands of frequencies of electromagnetic radiation.
Another object of the invention is to provide an off-axis filter having no expensive and fallible electronic or electromechanical parts.
Another object of the present invention is to provide a contrast enhancing component for a low level radiation imaging device which is simple in construction, inexpensive to manufacture, and capable of long life of useful service with minimum maintenance
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.